A part-time driving system which selectively switches between a four-wheel drive (4WD) mode and a two-wheel drive (2WD) mode and a full-time driving system which always drives four wheels are known as four-wheel driving systems for vehicles. In the part-time driving system, mode switching is according to manual operation by a driver. The full-time driving system has a center differential between the front wheels and rear wheels. The center differential permits the differential of the front wheels and the rear wheels to always drive the four wheels.
A standby driving system is known too as a four-wheel driving system other than the part-time driving system and full-time driving system. In this standby driving system, the main drive wheels of a vehicle are coupled to the engine without the intervention of the coupling and the sub-drive wheels are coupled to the engine via the coupling. The coupling force (engaging force) of the coupling is changed in accordance with the road conditions and driving state, thereby changing the torque distribution ratio DSUB/MAIN, which is the ratio of the torque to be transmitted to the sub-drive wheels from the engine to the torque to be transmitted to the main drive wheels from the engine.
In the standby driving system, a controller compares a wheel speed difference ΔN, which is the difference between the rotational speed of the main drive wheels and the rotational speed of the sub-drive wheels, with a predetermined threshold value. When the wheel speed difference ΔN exceeds the threshold value, the controller predicts that acceleration takes place and controls the coupling in such a way that the torque distribution ratio DSUB/MAIN increases, i.e., the torque distribution of the sub-drive wheels increases.
Even if the transmission is shifted down to accelerate the vehicle in the standby driving system, however, the controller cannot detect that acceleration is taking place until the wheel speed difference ΔN exceeds the threshold value. Therefore, there is a delay from the point at which shift-down has been performed to the point at which the increase in torque distribution ratio DSUB/MAIN starts.
Japanese Laid-Open Patent Publication No. 6-288415 discloses a mechanical coupling as the coupling that is used in a four-wheel driving system. Even if the mechanical coupling is adapted to a standby driving system, however, the beginning of the increase in torque distribution ratio DSUB/MAIN cannot be interlocked with the shift-down.